Well, I think I've found the source of the problem. I pullout the pcb (this time, all the way), so I could replace the neon, and was greeted with a mess on the back of the pcb. Looks like there was a rubber foot on the back, which was placed right under the C37,D25,D26,R42, which melted/disinigrated, and created a mess. (See pics)

I used some goof off to clean off all the black goo, then sprayed some Deoxit to remove the corrosion. The PCB is now clean again. I also cut out a new card to shield the PCB from the metal enclosure.

I also since I had the pcb out, I replaced R42 (200ohm/.5w).

Same symptoms still persist. Unit powers on, protect light off, no sound when taken off standby.

I also tried it with one of the connectors, disconnected, and the reading was the same on the other channel. FYI. I did notice on the front panel, that all the leds for one side were lit, while the connector was disconnected.

Looks like there could be a problem around Q9 (TIP48) in the PSU - this is part of the HV regulator (buy a couple). This might not be the only fault though, but I would start with trying to get that +170V rail back up first. Would also be a good idea to get a couple of the Q10 as well - MPSA42.

Hmm crackling often = bad solder joins, a good thing to do (regardless) is re-flow (remelt) all the solder joins particularly around the power supply (was suggested by MJMP and fixed my (crackling) B200s ).